1. Field of the Invention
The invention is based on an electrical resistor according to the preamble of patent claim 1. A resistor of this type identifies and limits short-circuit currents or overcurrents flowing in a load circuit. Only after this does a switch provided in the load circuit interrupt the limited current. The switch can therefore be designed for a small breaking power compared with the short-circuit power.
2. Discussion of Background
A current-limiting resistor of the afore-mentioned type is described, for example, in U.S. Pat. No. 5,313,184 A. Such a resistor contains two connection electrodes between which, connected in parallel with one another, a resistance body having PTC behavior and a varistor are arranged. The resistance body and the varistor make contact with one another via the entire insulation clearance between the two connection electrodes. This avoids local overvoltages in the resistance body and hence impermissibly high local thermal loading of the resistance body.
In order to increase the dielectric strength of this resistor, a plurality of resistors can be connected in series. Such an arrangement is relatively complicated since metal electrodes are arranged both between the individual resistance bodies and between the individual varistors. In the normal operating state of the resistor, the current is conducted through a series circuit of a plurality of resistance bodies having PTC behavior, between each of which bodies a metal electrode is arranged. The contact resistance between a metal electrode and the material of the resistance body is generally relatively high and, in the case of a typical resistor for current limiting tasks, having a total resistance of approximately 50 m.OMEGA., contributes just as much as the material of the resistance body to the total resistance. Furthermore, metal electrodes and the polymers which are usually used as material for the resistance body and are filled with a filler have different electrical conductivities and different thermal expansion coefficients. As a result, mechanical stresses may be produced in the interior of the resistor, which stresses may possibly impair the mechanical and electrical properties of said resistor.